The present invention relates to a method of preparing a silacycloalkane and more particularly to a method comprising adding an Si-substituted silacycloalkane to a suspension of lithium aluminum hydride in a glycol diether. The present invention also relates to a method of preparing a silicon carbide film using the silacycloalkane.
Methods of preparing silacycloalkanes are known in the art. For example, Nametkin et al. teaches the preparation of silacyclobutane by reduction of 1,1-dichloro-1-silacyclobutane in n-butyl ether at 55-60xc2x0 C. (Khim. Geterotsikl. Soedin., 1966, 4, 623).
Laane discloses, inter alia, the preparation of silacyclobutane in 60% yield by treatment of lithium aluminum hydride in ethyl ether with 1,1-dicholoro-1-silacyclobutane in n-butyl ether at xe2x88x925 to +5xc2x0 C. (J. Am. Chem. Soc. 1967, 89 (5), 1144-1147).
U.S. Pat. No. 4,973,723 to Cawthon et al. discloses the preparation of silacycloalkanes by reduction of halosilacycloalkanes with an alkylaluminum hydride. According to a preferred embodiment of the invention, the halosilacycloalkane is added to the alkylaluminum hydride under temperature and pressure conditions that cause the silacycloalkane to vaporize immediately after it is formed. The silacycloalkane product is vacuum distilled from the resultant mixture immediately upon formation, virtually eliminating the occurrence of further reactions of the silacycloalkane. According to the ""723 patent, improved yields of product are achieved by the described process.
Although the aforementioned references describe the preparation of silacycloalkanes by reduction of halosilacycloalkanes, there remains a need for a method of producing silacycloaklanes having high purity in high yield that is scaleable to a commercial manufacturing process.
The present invention is directed to a method of preparing a silacycloalkane having the formula: 
wherein n is 1, 2, or 3, comprising the steps of:
(A) adding a substituted silacycloalkane having the formula: 
wherein X1 is xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, or xe2x80x94OR1 and X2 is X1 or H, wherein R1 is C1-C8 hydrocarbyl and n is 1, 2, or 3, to a suspension of lithium aluminum hydride in a glycol diether at a temperature not greater than 50xc2x0 C. to form a mixture, wherein the glycol diether consists essentially of a linear arrangement of oxyalkylene units having formulae independently selected from xe2x80x94OCH2CH2xe2x80x94, xe2x80x94OCH2CH(CH3)xe2x80x94, and xe2x80x94OCH2CH(CH2CH3)xe2x80x94, and end-groups having the formulae xe2x80x94R2 and xe2x80x94OR2, wherein each R2 is independently C1-C8 alkyl, phenyl, or C1-C8 alkyl-substituted phenyl, provided the glycol diether has a normal boiling point of at least 85xc2x0 C. and a viscosity not greater than 3000 mm2/s at 25xc2x0 C.; and
(B) distilling the mixture under reduced pressure at a temperature not greater than 50xc2x0 C. to remove the silacycloalkane.
The method of the present invention produces silacycloalkanes having high purity in high yield. Importantly, the silacycloalkane can be readily and efficiently removed from the reaction mixture by distillation. This separation minimizes the occurrence of unwanted side reactions that can diminish purity and yield. Also, the silacycloalkane product is free of solvent, which can be deleterious in certain applications, particularly in the electronics field. Further, the method can be carried out economically with a stoichiometric amount or only slight excess of lithium aluminum hydride. Still further, the method can be scaled to a commercial manufacturing process.
The silacycloalkanes of the present invention can be used as coatings on solar panels and as precursors for synthesis of various sila drugs. Moreover, the silacycloalkanes can be used to prepare polycarbosilanes, which are useful as ceramic precursors.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
According to the present invention, a method of preparing a silacycloalkane having the formula: 
wherein n is 1, 2, or 3, comprising the steps of:
(A) adding a substituted silacycloalkane having the formula: 
wherein X1 is xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, or xe2x80x94OR1 and X2 is X1 or H, wherein R1 is C1-C8 hydrocarbyl and n is 1, 2, or 3, to a suspension of lithium aluminum hydride in a glycol diether at a temperature not greater than 50xc2x0 C. to form a mixture, wherein the glycol diether consists essentially of a linear arrangement of oxyalkylene units having formulae independently selected from xe2x80x94OCH2CH2xe2x80x94, xe2x80x94OCH2CH(CH3)xe2x80x94, and xe2x80x94OCH2CH(CH2CH3)xe2x80x94, and end-groups having the formulae xe2x80x94R2 and xe2x80x94OR2, wherein each R2 is independently C1-C8 alkyl, phenyl, or C1-C8 alkyl-substituted phenyl, provided the glycol diether has a normal boiling point of at least 85xc2x0 C. and a viscosity not greater than 3000 mm2/s at 25xc2x0 C.; and
(B) distilling the mixture under reduced pressure at a temperature not greater than 50xc2x0 C. to remove the silacycloalkane.
The substituted silacycloalkane of the present invention has the formula: 
wherein X1 is xe2x80x94F, xe2x80x94Cl, xe2x80x94Br, or xe2x80x94OR1 and X2 is X1 or H, wherein R1 is C1-C8 hydrocarbyl and n is 1, 2, or 3. The hydrocarbyl groups represented by R1 can have from 1 to 8 carbon atoms, alternatively from 1 to 4 carbon atoms. Hydrocarbyl groups having at least three carbon atoms can have a branched or an unranked structure. Examples of hydrocarbyl groups represented by R1 include, but are not limited to, alkyl such as methyl, ethyl, propyl, pentyl, and octyl; cycloalkyl such as cylcohexyl; alkenyl such as vinyl and allyl; and aryl such as phenyl, tolyl, xylyl, benzyl, and 2-phenylethyl. Preferably, R1 is alkyl and more preferably, R1 is methyl, ethyl, or propyl.
Examples of substituted silacycloalkanes include, but are not limited to, 1,1-difluoro-1-silacyclobutane, 1,1-dichloro-1-silacyclobutane, 1,1-dibromo-1-silacyclobutane, 1,1-dimethoxy-1-silacyclobutane, 1,1-difluoro-1-silacyclopentane, 1,1-dichloro-1-silacyclopentane, 1,1-dibromo-1-silacyclopentane, 1,1-dimethoxy-1-silacyclopentane, 1,1-difluoro-1-silacyclohexane, 1,1-dichloro-1-silacyclohexane, 1,1-dibromo-1-silacyclohexane, and 1,1-dimethoxy-1-silacyclohexane.
Furthermore, the substituted silacyclolalkane can be a single substituted silacyclolalkane or a mixture comprising two or more different substituted silacyclolalkanes.
The substituted silacycloalkanes wherein X1 and X2 are xe2x80x94Cl and n is 1 can be prepared using the method of Laane (J. Am. Chem. Soc. 1967, 89 (5), 1144-1147). According to this procedure, (3-chloropropyl)trichlorosilane (0.94 mol) in diethyl ether was added dropwise to a vigorously stirred suspension of magnesium powder (1.7 g-atoms) in ether (500 mL) over a period of 3 hours. The stirred mixture was heated for 72 hours and then allowed to cool to room temperature. The magnesium chloride and excess magnesium metal were removed by filtration and washed several times with ether. The filtrate and washings were combined and distilled to give 1,1-dichloro-1-silacyclobutane (61%) having a boiling point of 113-115xc2x0 C.
The substituted silacycloalkanes wherein X1 is Cl, X2 is H, and n is 1 can be prepared by substituting (3-chloropropyl)dichlorosilane for (3-chloropropyl)trichlorosilane in the above procedure.
Methods of preparing the substituted silacycloalkanes wherein X1 and X2 arexe2x80x94Cl and n is 2 (1,1-dichloro-1-silacyclobutane) or 3 (1,1-dichloro-1-silacyclopentane) are well known in the art. For example, these compounds can be prepared from the corresponding xcex1,xcfx89-dichloroalkanes or xcex1,xcfx89-dibromoalkanes and silicon tetrachloride by means of the Grignard reaction, as described in Organosilicon Compounds (Eaborn, C., Butterworths Scientific Publications: London, 1960, Chapter 13).
The substituted silacycloalkanes wherein X1 is xe2x80x94Cl, X2 is H, and n is 2 or 3 can be prepared by substituting trichlorosilane for silicon tetrachloride in the above procedure.
The substituted silacycloalkanes wherein X1 and X2 are xe2x80x94Br and n is 1, 2, or 3 can be prepared by treating the corresponding 1,1-dichloro-1-silacycloalkanes with molecular bromine. The exchange reactions between organosilicon halides and molecular halogens are well known in the art, as exemplified in Organosilicon Compounds (Eaborn, C., Butterworths Scientific Publications: London, 1960; pp 187-188).
The substituted silacycloalkanes wherein X1 is xe2x80x94Br, X2 is H, and n is 1, 2, or 3 can be prepared similarly by treating the corresponding 1-chloro-1-silacycloalkanes with molecular bromine.
The substituted silacycloalkanes wherein X1 and X2 are xe2x80x94F and n is 1, 2, or 3 can be prepared by treating the corresponding 1,1-dichloro- or 1,1-dibromo-1-silacycloalkanes with various metal fluorides. Examples of suitable metal fluorides include, but are not limited to, ZnF2, NaF, CsF, SbF5, and CoF2. The conversion of organochlorosilanes and organobromosilanes to the corresponding organofluorosilanes by treatment of the former with metal fluorides, ammonium fluoride, or sodium borofluoride in acetone, is well known in the art, as exemplified in Organosilicon Compounds (Eaborn, C., Butterworths Scientific Publications: London, 1960, pp 173-175).
The substituted silacycloalkanes wherein X1 is xe2x80x94F, X2 is H, and n is 1, 2, or 3 can be similarly prepared by treating the corresponding 1-chloro- or 1-bromo-1-silacycloalkanes with metal fluorides.
The substituted silacycloalkanes wherein X1 and X2 are OR1 and n is 1, 2, or 3, where R1 is as defined and exemplified above, can be prepared by contacting the corresponding 1,1-difluoro-, 1,1-dichloro-, or 1,1-dibromo-1-silacycloalkanes with at least two equivalents, based on the number of moles of silacycloalkane, of an alcohol or phenol. The reaction of the halosilacycloalkane and alcohol or phenol can be carried out in the presence of a base, such as pyridine or a tertiary amine, which combines with the liberated HCl. Preferably, the alcohol or phenol is substantially free of water. The reaction of the alcohol or phenol with the halosilacycloalkane can be carried out in an inert solvent such as ether or toluene. The reaction is frequently carried out using excess alcohol or phenol as the solvent.
The substituted silacycloalkanes wherein X1 is xe2x80x94OR1, X2 is H, and n is 1, 2, or 3 can be prepared similarly by treating the corresponding 1-chloro- or 1-bromo-1-silacycloalkanes with at least one equivalent, based on the number of moles of silacycloalkane, of an alcohol or phenol.
The lithium aluminum hydride is commercially available in solid form, e.g., a powder or pellets. Solutions of lithium aluminum hydride in various solvents are also commercially available. However, low boiling solvents such as diethyl ether, diglyme, ethylene glycol dimethyl ether, tetrahydrofuran, and toluene can distill with the silacycloalkane and contaminate the product. Such contamination can cause problems in certain applications, particularly in the electronics field, that require high purity. The lithium aluminum hydride is combined with the glycol diether to form a suspension.
The glycol diether of the present method consists essentially of a linear arrangement of oxyalkylene units having formulae independently selected from xe2x80x94OCH2CH2xe2x80x94, xe2x80x94OCH2CH(CH3)xe2x80x94, and xe2x80x94OCH2CH(CH2CH3)xe2x80x94, and end-groups having the formulae xe2x80x94R2 and xe2x80x94OR2, wherein each R2 is independently C1-C8 alkyl, phenyl, or C1-C8 alkyl-substituted phenyl. Examples of alkyl groups represented by R2 include, but are not limited to, methyl, ethyl, propyl, pentyl, hexyl, heptyl, and octyl. Examples of alkyl-substituted phenyl groups represented by R2 include, but are not limited to, tolyl, xylyl, benzyl, and 2-phenylethyl.
The glycol diether can be a dimer, trimer, oligomer, or polymer. The oxyalkylene units in the glycol diether can be connected head-to-tail, head-to-head, or tail-to-tail. The glycol diether can be a homopolymer or a copolymer, such as a random, alternating, periodic, or block copolymer. When the glycol diether contains at least two types of oxyalkylene units, the units can be sequentially arranged in any manner. Furthermore, the glycol diether can be a single glycol diether or a mixture comprising two or more different glycol diethers.
The glycol diether has a normal boiling point of at least 85xc2x0 C., alternatively at least 100xc2x0 C., alternatively at least 200xc2x0 C. When the normal boiling point of the glycol diether is less than 85xc2x0 C., the diether can distill with the silacycloalkane and contaminate the product.
The glycol diether is a liquid having a viscosity at 25xc2x0 C. not greater than 3000 mm2/s, alternatively not greater than 2000 mm2/s, alternatively not greater than 1000 mm2/s. When the viscosity of the glycol diether is greater than 3000 mm2/s, the reaction mixture is very viscous, resulting in less efficient admixing of the lithium aluminum hydride and substituted silacycloalkane. Nonuniform distribution of the co-reactants in the reaction mixture can result in formation of undesirable byproducts, diminishing the yield and purity of the silacycloalkane. Also, a highly viscous reaction mixture can hinder removal of the silacycloalkane by distillation.
Examples of glycol diethers include, but are not limited to, poly(ethylene glycol) dimethyl ether, diethylene glycol dibutyl ether, diethylene glycol diethyl ether, ethylene glycol diethyl ether, tetraethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and ethylene glycol dimethyl ether.
Methods of preparing glycol diethers are well known in the art; many of these compounds are commercially available.
The method of the present invention can be carried out in any standard reactor suitable for contacting lithium aluminum hydride with an organohalosilane. Suitable reactors include glass and Teflon-lined glass reactors. Preferably, the reactor is equipped with a means of agitation, such as stirring.
In the method of the present invention, the substituted silacycloalkane is necessarily added to the suspension of lithium aluminum hydride. Reverse addition, i.e., addition of lithium aluminum to the substituted silacycloalkane, produces a complex mixture of products that are not easily separated by distillation. Consequently, both the yield and purity of the silacycloalkane are diminished compared with the method of the present invention.
The rate of addition of the substituted silacycloalkane to the suspension of lithium aluminum hydride in the glycol diether is sufficiently slow to minimize distillation of the substituted silacylcoalkane from the reaction mixture in step (B) of the present method. Typically, the rate of addition is such that the distillate contains less than 20% (GC area %), alternatively less than 10% (GC area %) of the substituted silacycloalkane, as determined using the gas chromatography method in the Examples below. The rate of addition depends on factors such as reactor size, stirring efficiency, and temperature control. For example, the rate of addition can be from 1 to 4 mL/min., alternatively from 1 to 3 mL/min., alternatively from 2 to 3 mL/min, for a 100 mL reaction vessel equipped with an efficient means of stirring. Also, the rate of addition can be from 10 to 40 mL/min., alternatively from 10 to 30 mL/min., alternatively from 20 to 30 mL/min, for a 1 L reaction vessel equipped with an efficient means of stirring. When the rate of addition is too slow, the reaction time is unnecessarily prolonged. When the rate of addition is too fast, byproducts are formed, which reduce the yield and purity of the silacycloalkane. The optimum rate of addition can be easily determined by routine experimentation using the methods described in the Examples below.
The substituted silacycloalkane can be added to the suspension of lithium aluminum hydride in the glycol diether at a temperature not greater than 50xc2x0 C., alternatively not greater than 30xc2x0 C., alternatively not greater than 0xc2x0 C. At a temperature greater than 50xc2x0 C., the silacycloalkane may decompose. The minimum temperature of the reaction mixture is determined by the freezing points of the substituted silacycloalkane, glycol diether, and silacycloalkane product. The substituted silacycloalkane is added to the suspension of lithium aluminum hydride in the glycol diether at a temperature greater than the freezing point of any of the substituted silacycloalkane, glycol diether, and silacycloalkane product The substituted silacycloalkane can be added directly to the suspension of lithium aluminum hydride or diluted in a glycol diether and added to the mixture.
The mole ratio of lithium aluminum hydride to the substituted silacycloalkane can be from 0.5 to 3, alternatively from 0.5 to 1.2, alternatively from 0.5 to 0.7. When the mole ratio of lithium aluminum hydride to substituted silacycloalkane is less than 0.5, significant amount of the unsubstituted silacycloalkane fails to react. When the mole ratio is greater than 3, the cost of the process is increased unnecessarily.
The mole ratio of the glycol diether to the substituted silacycloalkane can be from 0.3 to 5, alternatively from 1 to 3, alternatively 1.5 to 2.5. When the mole ratio of glycol diether to substituted silacycloalkane is less than 0.3, the viscosity of the reaction mixture may be too high for efficient mixing. When the mole ratio is greater than 5, the cost of the process is increased unnecessarily.
The reaction mixture is distilled under reduced pressure at a temperature not greater than 50xc2x0 C. As used herein, the term xe2x80x9creduced pressure,xe2x80x9d means a pressure less than atmospheric pressure sufficient to volatilize the silacycloalkane and remove it from the reaction mixture. The particular pressure depends on the distillation temperature. For example, the pressure can be from 0.1 to 20 kPa at a temperature of from xe2x88x9220 to +25xc2x0 C., alternatively from 13 to 20 kPa at a temperature of from 5 to 25xc2x0 C. As the distillation temperature increases in the above ranges, the pressure required for volatilization of the silacycloalkane decreases. The optimum pressure at a particular temperature can be readily determined by routine experimentation.
Steps (A) and (B) of the present invention can be carried out in sequential order or simultaneously. For example, the substituted silacycloalkane can be added to the suspension of lithium aluminum hydride in the glycol diether followed by distillation of the reaction mixture under reduced pressure to remove the silacycloalkane. In this embodiment of the present invention, the addition of the substituted silacycloalkane, step (A), is preferably carried out in the substantial absence of atmospheric oxygen or moisture. This can be accomplished by purging the reactor with a dry inert gas, such as argon or nitrogen prior to introduction of the reactants and thereafter maintaining a blanket of the gas in the reactor.
Alternatively, the silacycloalkane can be added to the suspension of lithium aluminum hydride under reduced pressure with concomitant distillation of the reaction mixture to remove the silacycloalkane as it is formed, thus minimizing the occurrence of unwanted side reactions that can diminish purity and yield.
If desired, the silacycloalkane obtained by the method of the present invention can be further purified by at least one more distillation at a temperature less than 50xc2x0 C. under reduced pressure.
The method of the present invention produces silacycloalkanes having high purity in high yield. Importantly, the silacycloalkane can be readily and efficiently removed from the reaction mixture by distillation. This separation minimizes the occurrence of unwanted side reactions that can diminish purity and yield. Also, the silacycloalkane product is free of solvent, which can be deleterious in certain applications, particularly in the electronics field. Further, the method can be carried out economically with a stoichiometric amount or only slight excess of lithium aluminum hydride. Still further, the method can be scaled to a commercial manufacturing process.
The silacycloalkanes of the present invention can be used as coatings on solar panels and as precursors for synthesis of various sila drugs. Moreover, the silacycloalkanes can be used to prepare polycarbosilanes, which are useful as ceramic precursors.